HEAT stress in dry cows may not only lead to reduced production in the subsequent lactation, but it may also result in a less productive daughter, according to United States research.

University of Florida department of animal science chair Geoffrey Dahl was in Meningie lately as part of a national tour, discussing the often unseen impacts that heat can have on the long-term productivity of the herd.

Professor Dahl said traditionally, the concern about heat stress had been focused on lactating or transitioning cows but there were also consequences from allowing dry cows to get too hot.

He said research out of Florida, a traditionally hot and humid state, found the highest milk yields were in spring and the lowest in autumn, with most herds run on a total-mixed ration, which meant pasture quality was not a component.

“A lot more cows in autumn should be in early lactation, so there should be higher production then,” he said.

He said it could also be expected that the hottest months would coincide with reduced lactation, but instead it was offset, a couple of months after the hot period.

Prof Dahl said, in a controlled environment, they noticed a four litre to 5L reduction each day in milk produced by heat stressed cows, compared with those kept cool, which continued throughout the lactation.

Prof Dahl said there were indications a factor may be cows without heat stress had greater growth of epithelial cells in their mammary system, allowing greater capacity for milk production.

He said there were other factors, such as heat-stressed cows having reduced dry matter intake and less weight gain before the beginning of lactation.

He said heat stress could also have health impacts with heat-stressed dry cows having a higher incidence of mastitis and respiratory disease during their lactation, and were more likely to take longer to get pregnant.

Prof Dahl said a cow allowed to get heat-stressed at any point during their dry period showed the impacts later, but impacts occured on a gradient.

But while heat stress will just impact the next lactation for a cow, it is the gestating calf that may bear the greatest impact.

“If mum is heat-stressed, that calf is heat-stressed,” he said. “It does have effects early in life and when that animal is mature.”

Prof Dahl said calves born to heat-stressed dams generally calved in sooner, with a lower birthweight, weaning weight and immune status.

“The first lactation milk yields are about 5L a day less than cows born to cool dams, and there is no difference in the genetic potential or management,” he said.

“Even in subsequent lactations, they never catch up, and they pass that on to their offsprings.

“We’ve caused epigenetic effects permanently that won’t allow them to express their full genetic potential.”

He said there was the benefit that calves born to heat-stressed cows do have a greater potential to respond to high temperature stress events themselves, but this was a trade-off against lower productivity.

Prof Dahl says any attempt to keep a cow’s temperature down will have benefits.

He said temperatures in Florida, between April and November, could get up to 32 degrees Celsius, but there humidity was between 60 per cent and 100pc.

For herds in barns, cooling techniques included fans above stalls and water soakers near feed, running for 1.5 minutes every five minutes.

If mum is heat-stressed, that calf is heat-stressed. - GEOFFREY DAHL

“In our environment the humidity is so high, you have to cool the cow, not the environment,” he said. “In California and Mexico, dry cows are brought up to soakers two or three times a day.”

He said early studies on heat stress showed even something as simple as providing shade could help make a difference in keeping animals cooler.

He said it was important to provide good access to fresh, clean water, with many heat-stressed cows often consuming 50pc to 100pc more water than their cooler counterparts to try and regulate their body temperature.

“It doesn’t need to be as cool as in a shed with fans and soakers, the more a cow can be cooled off, the more positive it is for cows and for those calves,” he said.

He said there had been promising results from early studies into certain feed additives helping keep cows cooler.

Prof Dahl said one way to look for to check for heat stress was respiration. He said to count flank movements and if it occured more than 60 times within a one minute timeframe, that cow was likely to be heat-stressed.